Nucleophiles silyl

Copolymeriziation of polystyrene-bound dicyanoketene acetal (DCKA) and ethylene glycol dimethacrylate (EGDMA) yielded a polymer (41) with high n -acidity. It was found to be an effective and completely recyclable catalyst in the high yielding carbon-carbon bond-forming reaction of dimethylacetals with silylated nucleophiles (Scheme 4.26) [118]. 

Kobayashi et al. developed the polymeric scandium(lll)-catalyst (42) (PA-Sc-TAD) which promotes the three-component couphng reactions of aldehydes and aromatic amines with either alkenes to generate quinohnes or silylated nucleophiles to form y9-amino ketones, esters and nitriles. This methodology turned out to be highly efficient with regard to automated high throughput synthesis (Scheme 4.27) [119]. 

By 1989 Mukaiyama had already explored the behaviour of phosphonium salts as Lewis acid catalysts. It was possible to show that the aldol-type reaction of aldehydes or acetals with several nucleophiles and the Michael reaction of a,j3-unsatu-rated ketones or acetals with silyl nucleophiles gave the products in good yields with a phosphonium salt catalyst [116]. In addition, the same group applied bisphosphonium salts as shown in Scheme 45 in the synthesis of ]3-aminoesters [117]. High yields up to 98% were obtained in the reaction of A-benzylideneaniline and the ketene silyl acetal of methyl isobutyrate. Various analogues of the reaction parteers gave similar results. The bisphosphonium salt was found to be superior to Lewis acids like TiCl and SnCl, which are deactivated by the resulting amines. 

Bismuth-Catalyzed Addition of Silyl Nucleophiles to Carbonyl Compounds and Imines... 

Keywords Aza-Sakurai allylationreaction Bismuth triflate C-C bond formation Mannich-type reaction Mukaiyama aldol reaction /V-AI koxycaihony I am ino sulfones Silyl nucleophiles... 

As well as the Bingel reaction and its modifications some more reactions that involve the addition-elimination mechanism have been discovered. 1,2-Methano-[60]fullerenes are obtainable in good yields by reaction with phosphorus- [44] or sulfur-ylides [45,46] or by fluorine-ion-mediated reaction with silylated nucleophiles [47]. The reaction with ylides requires stabilized sulfur or phosphorus ylides (Scheme 3.9). As well as representing a new route to l,2-methano[60]fullerenes, the synthesis of methanofullerenes with a formyl group at the bridgehead-carbon is possible. This formyl-group can be easily transformed into imines with various aromatic amines. 

The reaction of Cjq with silylated nucleophiles [47] requires compounds such as silyl ketene acetals, silylketene thioacetals or silyl enol ethers. It proceeds smoothly and in good yields in the presence of fluoride ions (KF/18-crown-6) (Scheme 3.10). The advantage of the latter synthesis is the realization of the cyclopropanation under nearly neutral conditions, which complements the basic conditions that are mandatory for Bingel reactions. Reaction with similar silyl ketene acetals under photochemical conditions and without the use of F does not lead to methanofullerenes but to dihydrofullerene acetate [48]. 

Fe -exchanged montmorillonite, as well as Al and Sn -exchanged montmorillonite, not only works as an efficient solid acid catalyst in the addition reactions of carbonyl compounds using silylated nucleophiles, but also enables an easy work-up procedure which merely requires filtration to separate the products from the catalyst. 

Such unique acid catalysis of Fe-Mont, which is sometimes superior to the acid catalysis of superacid as seen in the present paper, is due possibly to the Me3Si cations as very strong Lewis acid sites that are formed initially from silylated nucleophilic reagents and the protons of Fe-Mont layer surfaces (Eq. 5) as recently suggested by the present authors [3]. 

In 1997, the first truly catalytic enantioselective Mannich reactions of imines with silicon enolates using a novel zirconium catalyst was reported [9, 10]. To solve the above problems, various metal salts were first screened in achiral reactions of imines with silylated nucleophiles, and then, a chiral Lewis acid based on Zr(IV) was designed. On the other hand, as for the problem of the conformation of the imine-Lewis acid complex, utilization of a bidentate chelation was planned imines prepared from 2-aminophenol were used [(Eq. (1)]. This moiety was readily removed after reactions under oxidative conditions. Imines derived from heterocyclic aldehydes worked well in this reaction, and good to high yields and enantiomeric excesses were attained. As for aliphatic aldehydes, similarly high levels of enantiomeric excesses were also obtained by using the imines prepared from the aldehydes and 2-amino-3-methylphenol. The present Mannich reactions were applied to the synthesis of chiral (3-amino alcohols from a-alkoxy enolates and imines [11], and anti-cc-methyl-p-amino acid derivatives from propionate enolates and imines [12] via diastereo- and enantioselective processes [(Eq. (2)]. Moreover, this catalyst system can be utilized in Mannich reactions using hydrazone derivatives [13] [(Eq. (3)] as well as the aza-Diels-Alder reaction [14-16], Strecker reaction [17-19], allylation of imines [20], etc. 

An IV-heterocyclic carbene (similar to that in Scheme 15) has proved to be an effective catalyst for the nucleophilic ring opening of IV-tosylaziridines by silylated nucleophiles (MesSiX, X = N3, Cl, I).45 Yields range from 89 to 99% for reaction at the least substituted carbon, except when a phenyl group on one of the carbons of the aziridine ring induces predominant attack at the benzyl carbon. The stereochemistry is consistent with the SN2 mechanism and THF was found to be the best solvent for the reaction. 

Harada, T. Mukaiyama, T. Trityl antimonate-catalyzed sequential reactions of epoxides with silylated nucleophiles. Rearrangement of epoxides and C-C or C-O bond forming nucleophilic reaction onto the intermediate carbonyl compounds. Bull. Chem. Soc. Jpn. 1993, 66, 882-891. 

Reaction of otfi-unsaturated acetals with silyl nucleophiles. This trityl salt is an efficient catalyst for reaction of the dimethyl acetal (2) of cinnamaldehyde with successive silyl nucleophiles to form y,5-unsaturated p-azido ketones. The azide group can be reduced to an amino group by the Staudinger reaction.1... 

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